High accuracy receiver forward and reflected path test injection circuit

ABSTRACT

There is disclosed an injection circuit for measuring radio frequency (RF) signals in an RF receiver for use in measuring the W impedance match of a receive antenna and for use in calibrating c) receiver gain, wherein an advantageous embodiment of the injection circuit comprises: 1) a circulator coupled to the receive antenna; 2) a directional coupler coupled to the circulator; 3) an injection source coupled to the circulator and to the directional coupler, wherein the injection source is capable of injecting a test RF signal into either the circulator or the directional coupler; and 4) a terminating switch for selectively enabling or disabling the transfer of a test RF signal from the injection source to either the circulator or the directional coupler. The circulator has a reverse isolation of at least 20 dB that significantly increases the accuracy of the measurements of the RF signals compared with the accuracy that may be achieved by prior art methods. The present invention obtains the received signal strength indicator (RSSI) measurements at any instantaneous temperature and operating channel and determines voltage standing wave ratio (VSWR) measurements.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present invention is related to that disclosed in U.S. patentapplication Ser. No. 09/475,604, filed Dec. 30, 1999, entitled “COMBINEDSYSTEM FOR CALIBRATING RECEIVER GAIN AND MEASURING ANTENNA IMPEDANCEMATCH AND METHOD OF OPERATION.” application Ser. No. 09/475,604 iscommonly assigned to the assignee of the present invention. Thedisclosure of the related patent application is hereby incorporated byreference in the present application as if fully set forth herein.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention is directed, in general, to wirelesscommunications systems and, more specifically, to radio frequency (RF)test injection circuits for measuring the antenna impedance match of areceive antenna and measuring receiver gain in a base station in awireless network.

BACKGROUND OF THE INVENTION

[0003] In order to increase the number of subscribers that can beserviced in a single wireless network, frequency reuse is maximized bymaking individual cell sites smaller and using a greater number of cellsites to cover the same geographical area. Accordingly, the greaternumber of base transceiver stations increases infrastructure costs. Tooffset this increased cost, wireless service providers continuallyimplement any improvements that may reduce equipment costs, maintenanceand repair costs, and operating costs, or that may increase servicequality and reliability, and the number of subscribers that the cellularsystem can service.

[0004] In many receivers characterization of forward path gain andcalibration of the received signal strength indicator (RSSI) signal arepresently accomplished with temperature compensation circuitry. Thetemperature compensation circuitry adapts to variations in gain,attenuation, and detector slopes over a range of temperatures andfrequencies. Because the characteristics of devices and components usedin the temperature compensation circuitry change with variations intemperature and frequency, the receivers must be calibrated andcharacterized at the time of manufacture. However, the characteristicsof the devices and components vary within different manufacturing lots.This means that the operating characteristics of the receivers must becontinuously monitored during the manufacturing process to detectchanges that occur as the manufacturing process progresses.

[0005] Therefore, the receiver circuitry must be characterized byanalyzing numerous individual receiver units during the manufacturingprocess in order to develop an accurate profile for the temperaturecompensation circuitry. After the receiver circuitry has beencharacterized, the characterization information must be stored in thememory of each of the individual receiver units. Because themanufacturing process produces component changes over a period of time,the receiver characterization process must be re-performed and theinformation in the memory of each of the individual receiver units mustbe updated.

[0006] There is therefore a need in the art for a receiver design thatdoes not require continual re-characterization of forward path gain andcontinual recalibration of Received Signal Strength Indicator (RSSI)during the manufacturing process.

[0007] After a base transceiver station (BTS) has been manufactured,wireless service providers use a variety of test equipment to monitorthe performance of the RF receiver and the RF transmitter in the BTSduring operation. The test equipment may monitor a variety of signalparameters in the RF transmitter, including adjacent channel power ratio(ACPR), spectral purity (including in-band and out-of-band spuriouscomponents), occupied bandwidth, RHO, frequency error, and code domainpower. The test equipment may also perform a variety of test functionsin the RF receiver, including testing and measuring the receive antennareturn loss and calibrating the receiver. Preferably, the signalparameters are remotely monitored from a central location, so that awireless service provider can avoid the expense of sending maintenancecrews into the field to test each BTS individually. Additionally, aremote monitoring system can detect the failure of an RF transmitter oran RF receiver nearly instantaneously.

[0008] Unfortunately, adding some types of test equipment (e.g.,spectrum analyzers) to a BTS significantly increases the cost of theBTS. In some cases, the cost of the test equipment may be greater thanthe cost of the BTS itself. As a result, wireless service providers maynot install any test equipment in the ETS. Alternatively, wirelessservice providers may install only a limited amount of test equipment totest only some of the functions of the BTS. The remaining functions mustbe monitored by maintenance crews using portable test equipment.

[0009] There is therefore a need in the art for inexpensive testequipment that may be implemented as part of the base station. Inparticular, there is a need for integrated test equipment that can reusesome of the existing circuitry in a base transceiver station. Moreparticularly, there is a need for integrated test equipment that can beused to measure the impedance match of a receive antenna and that can beused to calibrate the receiver gain.

[0010] Prior art RF test injection circuits have been used to measure RFsignals in an RF receiver in a base station in a wireless network forthe purpose of measuring the impedance match of a receive antenna and tocalibrate the receiver gain. A prior art injection circuit usuallycomprises a directional coupler that has an input coupled to a duplexerthat is coupled to an antenna array. The output of the directionalcoupler is coupled to a signal amplifier. Also coupled to thedirectional coupler is an injection source that is capable of injectinga test RF signal into the directional coupler.

[0011] When a prior art injection circuit of this type is used tomeasure the impedance match of a receive antenna, the injection sourceinjects a test RF signal into the directional coupler in the directionof the signal amplifier. Level detector circuitry that is coupled to thesignal amplifier measures the RSSI level of the test RF signal to obtaina first RSSI measurement of the test RF signal.

[0012] Then the injection source injects a test RF signal into thedirectional coupler in the direction of the duplexer that is coupled tothe antenna array. The test RF signal passes through the duplexer andhits the antenna array. RF signal energy that is not absorbed by theantenna array is reflected back through the duplexer and through thedirectional coupler to the signal amplifier and the level detectorcircuitry. The level detector circuitry coupled to the signal amplifiermeasures the RSSI level of the test RF signal to obtain a second RSSImeasurement of the reflected test RF signal. The level detectorcircuitry compares the two RSSI measurements to obtain a voltagestanding wave ratio (VSWR) that measures the impedance match of theantenna array.

[0013] One of the primary deficiencies of this prior art approach is thedifficulty of controlling the directivity of the directional coupler.This is because directional couplers are, capable of providing onlyapproximately 10 dB to 15 dB of reverse isolation between its inputsignal and its output signal. As a result, the directional coupler maytransfer a signal that is 10 dB to 15 dB below its output signal backthrough the duplexer to the antenna array. The relatively low level ofreverse isolation that is provided by the directional coupler means thata portion of the signal energy at the output of the directional couplerwill be transferred back through the duplexer to the antenna array andreflected back through the duplexer to the directional coupler. Thereflected energy adversely affects the RSSI measurements and causes anerroneous determination of the voltage standing wave ratio (VSWR). Thesame problem occurs when such a prior art injection circuit is used tocalibrate the receiver gain.

[0014] There is therefore a need in the art for an improved testinjection circuit for measuring radio frequency (RF) signals in an RFreceiver.

SUMMARY OF THE INVENTION

[0015] To address the deficiencies of the prior art described above, itis a primary object of the present invention to provide an improved testinjection circuit for measuring radio frequency (RF) signals in an RFreceiver. The improved test injection circuit of the present inventionmay be used to obtain highly accurate RF signal measurements todetermine the impedance match of a receive antenna. The improved testinjection circuit of the present invention may also be used to obtainhighly accurate RF signal measurements to calibrate receiver gain.

[0016] An advantageous embodiment of the improved test injection circuitof the present invention comprises: 1) a circulator coupled to an RFreceive antenna; 2) a directional coupler coupled to the circulator; 3)an injection source coupled to the circulator and to the directionalcoupler, wherein the injection source is capable of injecting a test RFsignal into either the circulator or the directional coupler; and 4) aterminating switch for selectively enabling or disabling the transfer ofa test RF signal from the injection source to either the circulator orthe directional coupler.

[0017] The circulator has a reverse isolation of at least 20 dB. This issignificantly greater than the 10 dB to 15 dB reverse isolation of aprior art directional coupler. The use of a circulator that has at least20 dB of reverse isolation significantly increases the accuracy of themeasurements of the RF signals compared with the accuracy that may beachieved by prior art methods.

[0018] The present invention is capable of obtaining receivedsignal-strength indicator (RSSI) measurements at any instantaneoustemperature and operating channel. The present invention is capable ofusing the RSSI measurements to obtain voltage standing wave ratio (VSWR)measurements for any instantaneous temperature and operating channel.

[0019] It is an object of the present invention to provide for use in anRF receiver unit a test injection circuit for accurately measuring RFsignals within the RF receiver unit.

[0020] It is another object of the present invention to provide for usein an RF receiver unit a test injection circuit for accurately measuringRF signals that comprises a circulator coupled to an antenna of the RFreceiver unit and an injection source coupled to the circulator that iscapable of injecting a test RF signal into the circulator.

[0021] It is also an object of the present invention to provide acirculator within the RF receive path of an RF receiver unit that has areverse isolation of at least 20 dB to increase the accuracy with whichRF signals may be measured in the RF receiver unit.

[0022] It is another object of the present invention to provide leveldetector circuitry within an RF receive unit to obtain highly accuratemeasurements of the received signal strength indicator of an RF signalwithin the RF receive unit.

[0023] It is still another object of the present invention to provide ahighly accurate method for calibrating the receiver gain of an RFreceive antenna.

[0024] It is also another object of the present invention to provide ahighly accurate method for measuring the impedance match of an RFreceive antenna.

[0025] The foregoing has outlined rather broadly the features andtechnical advantages of the present invention so that those skilled inthe art may better understand the detailed description of the inventionthat follows. Additional features and advantages of the invention willbe described hereinafter that form the subject of the claims of theinvention. Those skilled in the art should appreciate that they mayreadily use the conception and the specific embodiment disclosed as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. Those skilled in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the invention in its broadest form.

[0026] Before undertaking the DETAILED DESCRIPTION, it may beadvantageous to set forth definitions of certain words and phrases usedthroughout this patent document: the terms “include” and “comprise,” aswell as derivatives thereof, mean inclusion without limitation; the term“or,” is inclusive, meaning and/or; the phrases “associated with” and“associated therewith,” as well as derivatives thereof, may mean toinclude, be included within, interconnect with, contain, be containedwithin, connect to or with, couple to or with, be communicable with,cooperate with, interleave, juxtapose, be proximate to, be bound to orwith, have, have a property of, or the like; and the term “controller”means any device, system or part thereof that controls at least oneoperation, such a device may be implemented in hardware, firmware orsoftware, or some combination of at least two of the same. It should benoted that the functionality associated with any particular controllermay be centralized or distributed, whether locally or remotely.Definitions for certain words and phrases are provided throughout thispatent document, those of ordinary skill in the art should understandthat in many, if not most instances, such definitions apply to prior, aswell as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings,wherein like numbers designate like objects, and in which:

[0028]FIG. 1 illustrates an exemplary wireless network according to oneembodiment of the present invention;

[0029]FIG. 2 illustrates in greater detail an exemplary base station inaccordance with one embodiment of the present invention;

[0030]FIG. 3 illustrates a portion of an exemplary RF transceiver unitin accordance with an advantageous embodiment of the present inventioncomprising an injection circuit that uses a circulator and a directionalcoupler; and

[0031]FIG. 4 illustrates a flow diagram of a method used by anadvantageous embodiment of the present invention to determine voltagestanding wave ratio (VSWR) measurements.

DETAILED DESCRIPTION

[0032]FIGS. 1 through 4, discussed below, and the various embodimentsused to describe the principles of the present invention in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the invention. Those skilled in the artwill understand that the principles of the present invention may beimplemented in any suitably arranged wireless network.

[0033]FIG. 1 illustrates an exemplary wireless network 100 according toone embodiment of the present invention. The wireless telephone network100 comprises a plurality of cell sites 121-123, each containing one ofthe base stations, BS 101, BS 102, or BS 103. Base stations 101-103 areoperable to communicate with a plurality of mobile stations (MS)111-114. Mobile stations 111-114 may be any suitable cellular devices,including conventional cellular telephones, PCS handset devices,portable computers, metering devices, and the like.

[0034] Dotted lines show the approximate boundaries of the cells sites121-123 in which base stations 101-103 are located. The cell sites areshown approximately circular for the purposes of illustration andexplanation only. It should be clearly understood that the cell sitesmay have other irregular shapes, depending on the cell configurationselected and natural and man-made obstructions.

[0035] In one embodiment of the present invention, BS 101, BS 102, andBS 103 may comprise a base station controller (BSC) and a basetransceiver station (BTS). Base station controllers and base transceiverstations are well known to those skilled in the art. A base stationcontroller is a device that manages wireless communications resources,including the base transceiver station, for specified cells within awireless communications network. A base transceiver station comprisesthe RF transceiver unit, antennas, and other electrical equipmentlocated in each cell site. This equipment may include air conditioningunits, heating units, electrical supplies, telephone line interfaces,and RF transmitters and RF receivers, as well as call processingcircuitry. For the purpose of simplicity and clarity in explaining theoperation of the present invention, the base transceiver station in eachof cells 121, 122, and 123 and the base station controller associatedwith each base transceiver station are collectively represented by BS101, BS 102 and BS 103, respectively.

[0036] BS 101, BS 102 and BS 103 transfer voice and data signals betweeneach other and the public telephone system (not shown) viacommunications line 131 and mobile switching center (MSC) 140. Mobileswitching center 140 is well known to those skilled in the art. Mobileswitching center 140 is a switching device that provides services andcoordination between the subscribers in a wireless network and externalnetworks, such as the public telephone system. Communications line 131may be any suitable connection means, including a T1 line, a T3 line, afiber optic link, a network backbone connection, and the like. In someembodiments of the present invention, communications line 131 may beseveral different data links, where each data link couples one of BS101, BS 102, or BS 103 to MSC 140.

[0037] In the exemplary wireless network 100, MS 111 is located in cellsite 121 and is in communication with BS 101; MS 113 is located in cellsite 122 and is in communication with BS 102; and MS 114 is located incell site 123 and is in communication with BS 103. The MS 112 is alsolocated in cell site 121, close to the edge of cell site 123. Thedirection arrow proximate MS 112 indicates the movement of MS 112towards cell site 123. At some point, as MS 112 moves into cell site 123and out of cell site 121, a “handoff” will occur.

[0038] As is well known, the handoff procedure transfers control of acall from a first cell to a second cell. For example, if MS 112 is incommunication with BS 101 and senses that the signal from BS 101 isbecoming unacceptably weak, MS 112 may then switch to a BS that has astronger signal, such as the signal transmitted by BS 103. MS 112 and BS103 establish a new communication link and a signal is sent to BS 101and the public telephone network to transfer the on-going voice, data,or control signals through BS 103. The call is thereby seamlesslytransferred from BS 101 to BS 103. An “idle” handoff is a handoffbetween cells of a mobile device that is communicating in the control orpaging channel, rather than transmitting voice and/or data signals inthe regular traffic channels.

[0039]FIG. 2 illustrates in greater detail exemplary base station 101 inaccordance with one embodiment of the present invention. Base station101 comprises base station controller (BSC) 210 and base transceiverstation (BTS) 220. Base station controllers and base transceiverstations were described previously in connection with FIG. 1. BSC 210manages the resources in cell site 121, including BTS 220. BTS 220comprises BTS controller 225, channel controller 235, which containsrepresentative channel element 240, transceiver interface (IF) 245, RFtransceiver unit 250, antenna array 255 and impedance measurementcontroller 251.

[0040] BTS controller 225 comprises processing circuitry and memorycapable of executing an operating program that controls the overalloperation of BTS 220 and communicates with BSC 210. Under normalconditions, BTS controller 225 directs the operation of channelcontroller 235, which contains a number of channel elements, includingchannel element 240, that perform bi-directional communications in theforward channel and the reverse channel. A “forward” channel refers tooutbound signals from the base station to the mobile station and a“reverse” channel refers to inbound signals from the mobile station tothe base station. In an advantageous embodiment of the presentinvention, the channel elements operate according to a code divisionmultiple access (CDMA) protocol with the mobile stations in cell 121.Transceiver IF 245 transfers the bi-directional channel signals betweenchannel controller 235 and RF transceiver unit 250.

[0041] Impedance measurement controller 251, in conjunction withcircuitry located in RF transceiver unit 250, measures the receiver gainand the impedance match for the receive antenna portion of antenna array255. Portions of RF transceiver unit 250 and the operation of impedancemeasurement controller 251 are described below in greater detail inconnection with FIGS. 3 and 4.

[0042] Antenna array 255 transmits forward channel signals from RFtransceiver unit 250 to mobile stations in the coverage area of BS 101.Antenna array 255 also transfers to transceiver unit 250 reverse channelsignals received from mobile stations in the coverage area of ES 101. Inan advantageous embodiment of the present invention, antenna array 255is multi-sector antenna, such as a three sector antenna in which eachantenna sector is responsible for transmitting and receiving in a onehundred twenty degree (120°) arc of coverage area. Additionally, RFtransceiver unit 250 may contain an antenna selection unit to selectamong different antennas in antenna array 255 during both transmit andreceive operations.

[0043]FIG. 3 illustrates a portion of exemplary RF transceiver unit 250in accordance with an advantageous embodiment of the present invention.As will be fully described, the injection circuit of the presentinvention uses a circulator as a circuit element. An advantageousembodiment of the present invention comprises both a circulator and adirectional coupler. RF transceiver unit 250 comprises a circuit branch300 (shown in FIGURE 3) for receiving an RF signal. Circuit branch 300comprises antenna array 255, duplexer 310, circulator 370, directionalcoupler 320, low noise amplifier (LNA) 330, receive path circuitry 335,and level detector circuitry 340. Circuit branch 300 also comprisesinjection source 350 and single-pole double-throw (SPDT) terminatingswitch 360. Impedance measurement controller 251 enables and controlsthe measurement function of level detector circuitry 340.

[0044] Duplexer 310 filters the signal path to and from the antennaarray 255 as RF transceiver unit 250 transmits (forward channel) signalsand receives (reverse channel) signals. Duplexer 310 isolates thereceive signals in a receive signal frequency band (e.g., 1850-1910 MHz)from the transmit signals in a transmit signal frequency band (e.g.,1930-1990 MHz). Duplexer 310 permits the sharing of antenna array 255 bythe RF receiver portion and the RF transmitter portion of RF transceiverunit 250.

[0045] Receive (reverse channel) signals from antenna array 255 passthrough duplexer 310 to circulator 370. In this exemplary advantageousembodiment of the present invention, circulator 370 is a device that hasthree terminals. The three terminals are a first input terminal (coupledto duplexer 310) and a second input terminal (coupled to terminatingswitch 360) and an output terminal (coupled to directional coupler 320).When circulator 370 receives a signal at one of its terminals,circulator 370 transfers the signal to an adjacent terminal. Somecirculators are designed to circulated a signal in a clockwisedirection. Some circulators are designed to circulate a signal in acounterclockwise direction. In the exemplary advantageous embodiment ofthe present invention shown in FIG. 3 circulator 370 circulates a signalin a clockwise direction.

[0046] When circulator 370 receives an RF signal from duplexer 310 onthe first input terminal of circulator 370, then circulator 370transfers the signal to directional coupler 320. At the same time,circulator 370 transfers to duplexer 310 any signal that circulator 370receives on its second input terminal from single-pole double-throw(SPDT) terminating switch 360. As shown in FIG. 3, the second inputterminal of circulator 370 is connected to one output of terminatingswitch 360. As described more fully below, the signal from terminatingswitch 360 originates from injection source 350 and is used to measure areflected signal from antenna array 255.

[0047] The signals that circulator 370 transfers to directional coupler320 pass through directional coupler 320. Directional coupler 320combines the signals from circulator 370 with the signals, if any, fromthe other input of directional coupler 320 (i.e., the input ofdirectional coupler 320 that is coupled to terminating switch 360 andinjection source 350). The resulting signals then pass to low noiseamplifier (LNA) 330. Low noise amplifier 330 amplifies the signals fromdirectional coupler 320 and transfers the amplified signals to receivepath circuitry 335. The output of receive path circuitry 335 is coupledto transceiver IF 245. Level detector circuitry 340 is coupled toreceive path circuitry 335. Impedance measurement controller 251 iscoupled to level detector circuitry 340 and is capable of enabling andcontrolling the measurement function of level detector circuitry 340.

[0048] In this advantageous embodiment of the present invention, theinjection circuitry comprises injection source 350 that is coupled tocirculator 370 and to directional coupler 320 by a single-poledouble-throw (SPDT) terminating switch 360. The output of injectionsource 350 is provided as an input to terminating switch 360.Terminating switch 360 may transfer the injection signal from theinjection source 350 either to the second input terminal of circulator370 or to the coupled-input of directional coupler 320.

[0049] Impedance measurement controller 251 is capable of controllingterminating switch 360. Impedance measurement controller 251 selectivelyenables and disables the output from injection source 350,by controllingthe position of terminating switch 360 depending upon the type of RFsignal measurement to be performed. When impedance measurementcontroller 251 causes terminating switch 360 to close toward the rightside of terminating switch 360 (as shown in FIG. 3), injection source350 provides an injection signal to directional coupler 320. Whenimpedance measurement controller 251 causes terminating switch 360 toclose toward the left side of terminating switch 360 (as shown in FIG.3), injection source 350 provides an injection signal to circulator 370.The injection signal is an RF signal with a frequency that is preferablyin the central portion of the frequency operating range of the RFreceiver portion of RF transceiver unit 250.

[0050] As previously mentioned, impedance measurement controller 251enables and controls the measurement function of level detectorcircuitry 340. The level detector circuitry 340 provides AGC and RSSIlevel detection for use by impedance measurement controller 251. In anadvantageous embodiment of the present invention, level detectorcircuitry 340 uses AGC detectors in transreceiver IF 245 for detectingAGC levels, rather than providing a separate AGC detector formeasurement purposes. If measurements are not being performed, impedancemeasurement controller 251 disables the output of injection source 350.When injection source 350 is disabled, no signal is being injected intoRF transceiver unit 250 for measurement purposes.

[0051] The injection signal from injection source 350 may be used tomeasure the return loss of antenna array 255, as well as measureparameters associated with the performance of the RF receiver portion ofRF transceiver unit 250. When injection source 350 is connected todirectional coupler 320 through terminating switch 360, directionalcoupler 320 provides an output for level detector circuitry 340 that isa combination of the injection signal from injection source 350 and theRF receive signal, if any. The output signal from directional coupler320 is used for measuring the RSSI level of signals in the RF receivepath for use in determining the return loss for antenna array 255 or forcalibrating the RF receiver of RF transceiver unit 250.

[0052] In this advantageous embodiment of the invention, the return lossfor antenna array 255 may be measured. In the first step of themeasurement process, impedance measurement controller 251 causesterminating switch 360 to close toward the right side of terminatingswitch 360 (as shown in FIG. 3) to cause a first injection signal to betransferred to directional coupler 320. First injection signal iscoupled in the direction of low noise amplifier (LNA) 330, receive pathcircuitry 335, and level detector circuitry 340. For convenience, thisdirection will be referred to as the “receiver forward path.” The firstinjection signal is treated as a normal input signal which goes throughreceive path circuitry 335 to automatic gain control (AGC) circuit (notshown). The AGC circuit controls the gain of the first injection signalin accordance with well known AGC principles. Level detector circuitry340 then determines the RSSI level of the first injection signal andrecords the RSSI level of the first injection signal in impedancemeasurement controller 251.

[0053] In the second step of the measurement process, impedancemeasurement controller. 251 causes terminating switch 360 to closetoward the left side of terminating switch 360 (as shown in FIGURE 3) tocause a second injection signal to be transferred to the second inputterminal of circulator 370. The second injection signal is identical tothe first injection signal and has the same frequency and amplitude asthe first injection signal. Circulator 370 transfers the secondinjection signal to duplexer 310. The second injection signal travelsthrough duplexer 310 and hits antenna array 255. Energy that is notabsorbed by antenna array 255 reflects back through duplexer 310, aroundcirculator 370 and down circuit branch 300 in the direction of thereceiver forward path through low noise amplifier 330. The reflectedsecond injection signal is treated as a normal input signal which goesthrough receive path circuitry 335 to an automatic gain control (AGC)circuit (not shown). The AGC circuit controls the gain of the secondinjection signal in accordance with well known AGC principles. Leveldetector circuitry 340 then determines the RSSI level of the secondinjection signal and records the RSSI level of the second injectionsignal in impedance measurement controller 251.

[0054] A software algorithm in impedance measurement controller 251compares the two recorded RSSI levels and determines a voltage standingwave ratio (VSWR) measurement. Impedance measurement controller 251 thenstores the result as the return loss measurement for antenna array 255.

[0055] The injection circuit of the present invention will work even ifthere is no duplexer 310 in circuit branch 300. That is, the injectioncircuit that comprises circulator 370 will work in an RF receiver thatdoes not include a duplexer. Duplexer 310 is used in RF transceiverunits that are capable of both transmitting and receiving RF signals. Inan RF receiver without an RF transmitter duplexer 310 will not bepresent.

[0056] The use of circulator 370 in the advantageous embodiment of thepresent invention facilitates the control of the directivity of theinjected signals. Specifically, circulator 370 is capable of providingat least approximately 20 dB of reverse isolation between its inputsignal and its output signal. As a result, circulator 370 may transfer asignal that is at least approximately 20 dB below its output signal backthrough duplexer 310 to antenna array 255. The level of reverseisolation provided by circulator 370 exceeds the level of reverseisolation provided by a directional coupler such as directional coupler320.

[0057] The inclusion of circulator 370 in circuit branch 300 improvesthe accuracy in the measurements of signals from antenna array 255 byincreasing the level of isolation between duplexer 310 (or antenna array255) and low noise amplifier 330 with respect to the level of isolationavailable in prior art designs. The relatively high level of reverseisolation provided by circulator 370 (compared to the level of reverseisolation provided by directional couplers) means that in the presentinvention it is less likely that a portion of the signal energy at theoutput of circulator 370 will be transferred back through duplexer 310to antenna array 255 and reflected back through duplexer 310 tocirculator 370 and into the receiver forward path direction. Anyadditional energy that is reflected back into the receiver forward pathdirection adversely affects the RSSI measurements and causes anerroneous determination of the voltage standing wave ratio (VSWR). Inthe present invention, the use of circulator 370 in conjunction withdirectional coupler 320 reduces the levels of the additional reflectedenergy and significantly improves the accuracy of the RF signalmeasurements.

[0058] Impedance measurement controller 251 initiates an RSSIcalibration by enabling injection source 350 and injecting an injectionsignal through terminating switch 360 to directional coupler 320.Directional coupler 320 combines the injection signal from terminatingswitch 360 with an RF receive signal from circulator 370, if any, andtransfers the resulting signal to low noise amplifier 330. The resultingsignal is treated as a normal input signal which goes through receivepath circuitry 335 to an automatic gain control (AGC) circuit (notshown). The AGC circuit controls the gain of the resulting signal inaccordance with well known AGC principles. Level detector circuitry 340then determines the RSSI level of the resulting signal and records theRSSI level of the resulting signal in impedance measurement controller251.

[0059] Impedance measurement controller 251 uses the RSSI measurement tooffset, an existing RSSI curve that is stored in impedance measurementcontroller 251. The amount of offset represents the variation in RFreceiver gain caused by component performance variations due totemperature and frequency. Impedance measurement controller 251 storesthe offset and the RSSI measurement for use in calibrating the RFreceiver gain.

[0060] During the RSSI calibration measurement, the presence ofcirculator 370 provides at least 20 dB of reverse isolation in circuitbranch 300 and serves to reduce the amount of signal energy that reachesduplexer 310. Any signal energy that reaches circulator 370 fromdirectional coupler 320 is transferred toward terminating switch 360 andis not reflected back into the receiver forward path direction.

[0061] Prior art injection circuitry without circulator 370 allowssignal energy to travel toward duplexer 310. Depending upon the amountof antenna load, the signal energy reflects off the antenna load andback into the receiver forward path direction. The reflected signalenergy that is allowed to occur in prior art injection circuitryadversely affects the signal measurement process and the accuracy of themeasured signals.

[0062] The presence of circulator 370 in circuit branch 300 reduces theamount of signal energy that is reflected back into the receiver forwardpath direction. In this manner, circulator 370 increases the accuracy ofthe measurements of the signals over the accuracy that may be achievedby prior art methods that use injection circuitry.

[0063] The present invention provides an improved apparatus and methodfor injecting a test signal (i.e., an injection signal) into an RFreceiver to obtain the corresponding RSSI measurement at anyinstantaneous temperature and operating channel. The ability to obtainsuch RSSI measurements eliminates the need for the characterization andcompensation circuitry employed by prior art methods that do not useinjection circuitry.

[0064] In order to further increase the accuracy of signal measurements,impedance measurement controller 251 may make sure that no incoming RFreceive signal is present from duplexer 310 (or antenna array 255 ifthere is no duplexer 310) that would interfere with the measurement ofthe injection signal prior to the initiation of the measurement process.In an exemplary advantageous embodiment of the present invention,injection source 350 provides an 1880 MHz injection signal that fallsdirectly in the center of the assigned forward channel frequency range(1880 MHz±660 kHz) with a power level that is high enough to bediscernable above expected noise levels. The use of an injection signalin this frequency range allows BTS 101 to perform voltage standing waveratio (VSWR) measurements and calibration while also handling normalcommunications traffic.

[0065] An advantageous embodiment of the injection circuitry of thepresent invention has been described that comprises circulator 370 anddirectional coupler 320. It is possible, however, to use circulator 370without directional coupler 320. In this embodiment of the presentinvention an injection signal is transferred to circulator 370 and thentransferred by circulator 370 to duplexer 310. The injection signalpasses through duplexer 310 and hits antenna array 255. Energy that isnot absorbed by antenna array 255 reflects back down circuit path 3.00in the direction of the receiver forward path through low noiseamplifier 330. Level detector circuitry 340 then obtains the RSSImeasurement as previously described. The presence of directional coupler320 is not required to obtain RSSI measurements for the reflectedinjection signal. Directional coupler 320 is used to obtain a referencefor the voltage standing wave ratio (VSWR) measurement.

[0066]FIG. 4 illustrates an exemplary flow diagram 400 that describesthe operation of the exemplary advantageous embodiment of the presentinvention in RF transceiver unit 250 for measuring the return loss ofantenna array 255. Initially, impedance measurement controller 251enables injection source 350 to generate a first injection signal(process step 405). The frequency of the first injection signal ispreferably at the center frequency of the RF receiver operating range.Next, impedance measurement controller 251 enables terminating switch360 to transfer the first injection signal from injection source 350through terminating switch 360 to one input of directional coupler 320(process step 410).

[0067] Directional coupler 320 combines the first injection signal frominjection source 350 with the RF receive signal from circulator 370 andtransfers the combined signal to level detector circuitry 340 via lownoise amplifier 330 (process step 415). Level detector circuitry 340 inRF transceiver unit 250 receives the amplified signal from low noiseamplifier 330, uses the existing AGC detector of transceiver IF 245 toautomatically adjust the gain of the signal from low noise amplifier330, and measures the resultant RSSI level for the first injectionsignal and records the resultant RSSI level for the first injectionsignal in impedance measurement controller 251 (process step 420).

[0068] Next, impedance measurement controller 251 enables terminatingswitch 360 to send a second injection signal from injection source 350to the second input of circulator 370. The second injection signal hasthe same frequency and amplitude as the first injection signal.Circulator 370 then passes the second injection signal through duplexer310 to antenna array 255 (process step 425). Next, duplexer 310transfers the reflected second injection signal from antenna array 255to level detector circuitry 340 through circulator 370, directionalcoupler 320, and low noise amplifier 330 (process step 530). Leveldetector circuitry 340 in RF transceiver unit 250 receives the amplifiedsignal from low noise amplifier 330, uses the existing AGC detector oftransceiver IF 245 to automatically adjust the gain of the signal fromlow noise amplifier 330, and measures the resultant. RSSI level for thereflected second injection signal and records the resultant RSSI levelfor the reflected second injection signal in impedance measurementcontroller 251 (process step 435). Impedance measurement controller 251uses a software algorithm to compare the two measured RSSI levels anddetermines a voltage standing wave ratio (VSWR) measurement (processstep 440).

[0069] Although the present invention has been described in detail,those skilled in the art should understand that they can make variouschanges, substitutions and alterations herein without departing from thespirit and scope of the invention in its broadest form.

What is claimed is:
 1. For use in an RF receiver unit comprising an RFreceive path that is capable of being coupled to an antenna that iscapable of receiving an RF signal, an injection circuit for measuring RFsignals in said RF receive path comprising: a circulator coupled to saidantenna and coupled to said RF receive path; and an injection sourcecoupled to said circulator, wherein said injection source is capable ofinjecting a test RF signal into said circulator.
 2. The injectioncircuit as set forth in claim 1 further comprising a switch forselectively enabling and disabling the transfer of said test RF signalfrom said injection source to said circulator.
 3. The injection circuitas set forth in claim 1 wherein said circulator has a reverse-isolationof at least 20 dB.
 4. The injection-circuit as set forth in claim 1wherein said RF receive path comprises an amplifier and level detectorcircuitry coupled to said amplifier, said level detector circuitrycapable of measuring the received signal strength indicator of an RFsignal in said RF receive path.
 5. The injection circuit as set forth inclaim 1 wherein said RF receive path comprises an amplifier and leveldetector circuitry coupled to said amplifier and automatic gain controlcircuitry coupled to said level detector circuitry, said automatic gaincontrol circuitry capable of controlling the gain of an RF signal insaid RF receive path.
 6. The injection circuit as set forth in claim 1further comprising a duplexer coupled between said antenna and saidcirculator.
 7. The injection circuit as set forth in claim 1 furthercomprising a directional coupler coupled to said circulator and to saidRF receive path.
 8. The injection circuit as set forth in claim 7wherein said injection source is coupled to said directional coupler,and wherein said injection source is capable of injecting a test RFsignal into said directional coupler.
 9. The injection circuit as setforth in claim 8 further comprising a switch coupled to said circulatorand coupled to said directional coupler for selectively enabling anddisabling the transfer of test RF signals from said injection source tosaid circulator and from said injection source to said directionalcoupler.
 10. The injection circuit as set forth in claim 9 furthercomprising an impedance measurement controller coupled to said switch,said impedance measurement controller capable of causing said switch toselectively enable and disable the transfer of test RF signals from saidinjection source to said circulator and from said injection source tosaid directional coupler.
 11. The injection circuit as set forth inclaim 9 wherein said circulator has a reverse isolation of at least 20dB.
 12. The injection circuit as set forth in claim 9 wherein said RFreceive path comprises an amplifier and level detector circuitry coupledto said amplifier, said level detector circuitry capable of measuringthe received signal strength indicator of an RF signal in said RFreceive path.
 13. The injection circuit as set forth in claim 9 whereinsaid RF receive path comprises an amplifier and level detector circuitrycoupled to said amplifier and automatic gain control circuitry coupledto said level detector circuitry, said automatic gain control circuitrycapable of controlling the gain of an RF signal in said RF receive path.14. The injection circuit as set forth in claim 9 further comprising aduplexer coupled between said antenna and said circulator.
 15. A methodfor calibrating the RF receiver gain of an RF receive antenna that iscoupled to an RF receive path comprising the steps of: generating a testRF signal in an RF signal injection source; transferring said test RFsignal from said RF signal injection source to a circulator that iscoupled to said antenna and that is coupled to said RF receive path;transferring said test RF signal from said circulator to said antenna;measuring the received signal strength indicator of a portion of saidtest RF signal that is reflected from said antenna into said RF receivepath; obtaining an offset to an existing received signal strengthindicator curve using said measurement of received signal strengthindicator, wherein said offset represents variations in said RF receivergain that are caused by component performance variations due totemperature and frequency; and using said offset to calibrate said RFreceiver gain.
 16. The method as set forth in claim 15 furthercomprising the steps of: transferring said test RF signal from saidcirculator to a duplexer coupled to said antenna and coupled to saidcirculator; and measuring the received signal strength indicator of aportion of said test RF signal that is reflected from said antenna andfrom said duplexer into said RF receive path.
 17. A method for measuringthe antenna loss of an RF receive antenna that is coupled to an RFreceive path comprising the steps of: generating a first test RF signalin an RF signal injection source; transferring said first test RF signalfrom said RF signal injection source to a directional coupler, whereinone end of said directional coupler is coupled to a circulator that iscoupled to said antenna and the other end of said directional coupler iscoupled to said RF receive path; transferring said first test RF signalfrom said directional coupler to level detector circuitry within said RFreceive path; measuring within said level detector circuitry thereceived signal strength indicator of said first test RF signal fromsaid directional coupler to obtain a first received signal strengthindicator measurement; generating a second test RF signal in said RFsignal injection source that is identical to said first test RF signal;transferring said second test RF signal from said RF signal injectionsource to said circulator that is coupled to said antenna and that iscoupled to said directional coupler; transferring said second test RFsignal from said circulator to said antenna; measuring within said leveldetector circuitry the received signal strength indicator of a portionof said second test RF signal that is reflected from said antennathrough said circulator and through said directional coupler and to saidlevel detector circuitry to obtain a second received signal strengthindicator measurement; and comparing said first received signal strengthindicator measurement with said second received signal strengthindicator measurement to determine the voltage standing wave ratio forsaid antenna.
 18. The method as set forth in claim 17 further comprisingthe steps of: transferring said second test RF signal from saidcirculator to a duplexer coupled to said antenna and coupled, to saidcirculator; and measuring within said level detector circuitry thereceived signal strength indicator of a portion of said second test RFsignal that is reflected from said antenna through said duplexer andthrough said circulator and through said directional coupler and to saidlevel detector circuitry to obtain a second received signal strengthindicator measurement.
 19. The method as set forth in claim 17 furthercomprising the step of: combining with said first test RF signal fromsaid directional coupler an RF receive signal from said antenna.
 20. Themethod as set forth in claim 19 further comprising the step of:combining with said second test RF signal from said circulator an RFreceive signal from said antenna.